Flexible fingerprint sensor

ABSTRACT

A flexible pressure sensor has a first set of substantially parallel conductors in the x direction, a second set of substantially parallel conductors in the y direction, and a composite material disposed between the first set and second set of conductors. The composite material is capable of returning to substantially its original dimensions on release of pressure. The composite material includes conductive particles at least partially embedded in an elastomeric layer that have no relative orientation and are disposed within the elastomeric layer for electrically connecting the first set and second set of conductors in the z direction under application of sufficient pressure there between.

PRIORITY CLAIM AND CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is continuation application of U.S. patentapplication Ser. No. 11/877,662 filed on Oct. 23, 2007 and entitled“Flexible Fingerprint Sensor”, which is a non-provisional application ofU.S. provisional patent application Ser. No. 60/853,796 filed on Oct.23, 2006 and entitled “Flexible Fingerprint Sensor.” This patentapplication is also related to PCT patent application numberPCT/US2007/082309 filed on Oct. 23, 2007 and entitled “FlexibleFingerprint Sensor.” All of the foregoing patent applications are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to the field of pressure sensorsand, more particularly, to a flexible fingerprint sensor.

BACKGROUND OF THE INVENTION

The events of Sep. 11, 2001 and the explosion in “identity theft” haveraised the need for reliable personal identification. There exists aneed in society to positively identify, or at least, to authenticate aperson's identity in the course carrying out certain transactions. Thesetransactions range from purchasing items with a credit card to boardingcommercial airplanes. This need is currently met superficially throughlooking at someone's driver's license, asking for a simple PIN code, andother similar but inadequate means. A more reliable means is to use abiometric identifier as one criterion to authorize the transaction.

Recently, a slew of biometric identification methods have been proposed,including iris scans, DNA matching, heart-beat patterns, and many otheresoteric approaches. On the other hand, the human fingerprint has beenan accepted method for identification and matching of persons for wellover 100 years, and has withstood the test of time through numerouslegal and court cases. In order for fingerprint identification to bewidely used on hand-held devices, credit and debit cards, andidentification badges and cards, the fingerprint sensor must be lowcost, thin, mechanically conforming, mechanically flexible, and provideadequate resolution. Prior art fingerprint sensors fail to satisfy allof these requirements as evidenced by the lack of fingerprint sensors oncommercially produced hand-held devices, credit and debit cards, andidentification badges and cards.

There are several methods for capturing fingerprints: physical(inkpads), optical, electronic optical scan, capacitive and specializedmaterials. The inkpad method is the oldest method whereby the finger'spattern is transferred to a paper record using ink and finger pressure.The optical method uses a film or electronic camera to capture anoptical image of the fingerprint. This is typically used in criminalinvestigations to document latent fingerprints from a crime scene. Theelectronic optical scan method involves the scanning of a fingerprintthrough its reflection of light, which is transferred directly from thesubject finger to the opto-electronic sensor. This method has been inuse in numerous government applications, including the issuance ofdrivers' licenses. These sensors must be flat and rigid. As a result,they are not suitable for use on most hand-held devices, credit anddebit cards, and identification badges and cards.

The capacitive method is a more recent technique that is based on themeasurement of the varying electric field produced by the fingerprintridges. One example of this method is disclosed in U.S. Pat. No.7,250,774. The sensor is a made using a semiconductor chip. The physicalsize of the sensor chip is typically at least as large as the area ofthe fingerprint to be captured. As a result, semiconductor-based sensorscannot be low cost because of the process complexities and the lownumber of sensors that can be made from a single silicon wafer. Hence,these devices cannot take advantage of the cost reduction curve due tointegration that is characteristic of other semiconductor devices (humanfingers are not getting smaller). Finally, these semiconductor sensorsare made on a rigid substrate that requires a strong and flat surfacefor mounting in order to prevent the sensor from fracturing. Althoughmost semiconductor sensors are inherently thin, the requirement for arigid mounting substrate adds to the overall mounting platformthickness. These sensors must also be flat. As a result of theserequirements and the cost of the sensor, they are not suitable for useon most hand-held devices, credit and debit cards, and identificationbadges and cards.

Various semiconductor-based sensors have been developed to use smallersensor arrays (a small number of sensing lines in one dimension acrossthe sensor) to capture images as the finger is swiped across the sensorthat are assembled together create an image of the fingerprint (e.g.,U.S. Pat. No. 6,580,816). The sensors require sophisticated processingsoftware/hardware to assemble the individual images together into acomplete image of a fingerprint. Although this design addresses the costissue, it makes the mechanical reliability of this arrangement evenworse than the more rigid two-dimensional array. These sensors must alsobe flat. As a result of these requirements and the cost of the sensor,they are not suitable for use on most hand-held devices, credit anddebit cards, and identification badges and cards.

More recently, semiconductor based fingerprint sensors have beenmanufactured on flexible substrates, using amorphous silicon or otherresistive material deposited on top of a variety of substrate materials(e.g., U.S. Pat. No. 6,680,485 and U.S. Published Patent Application No.20060273417). Some of these devices are active sensors that are costlyto manufacture and have high power requirements. Other devices usepolymeric transistors that are unreliable in some environmentalconditions. U.S. Pat. No. 6,680,485 attempts to address the flexibilityand cost constraints required to put fingerprint sensors on hand-helddevices, credit and debit cards, and identification badges and cards.

A number of other techniques have been proposed to manufactureelectronic fingerprint sensors, although none of these are in widespreaduse commercially. Some of these techniques include the use ofelectro-luminescent materials (e.g., U.S. Pat. No. 7,248,298),piezo-electric materials (e.g., U.S. Pat. No. 5,515,738),magneto-resistive materials (e.g., U.S. Pat. No. 7,077,010), and others.These sensors are not suitable for use on hand-held devices, credit anddebit cards, and identification badges and cards.

Unlike fingerprint sensors, low resolution pressure sensors have beendeveloped using lower cost sensor elements (e.g., U.S. Pat. Nos.5,033,291 and 6,964,205) and composite materials (e.g., U.S. Pat. Nos.4,644,101 and 6,915,701 and 7,059,203 and 7,080,562 and 7,260,999).These sensors are typically used for pressure responsive input devices(e.g., U.S. Pat. No. 4,644,101), pressure distribution sensors (e.g.,U.S. Pat. Nos. 5,033,291 and 6,964,205), and shear force sensors (e.g.,U.S. Pat. No. 6,915,701).

U.S. Pat. No. 4,644,101 discloses a position sensor assembly whichcomprises a composite layer medium including electrically conductivemagnetic particles in a nonconductive matrix material. The particles arealigned into chains extending across the thickness of the layer, and thechains include a non-conductive gap which is bridged upon application ofsufficient pressure. The medium is sandwiched between sheet electrodessuch that electrical measurements can be taken at specific points at theperiphery of the sensor to determine where the pressure is beingapplied. Similarly, U.S. Pat. No. 6,915,701 discloses the use ofnon-random patterns of conductive particles to form columns havingdefined orientations in a non-conductive matrix. Although these sensorsprovide signals that are indicative of the position of a locally appliedpressure, it cannot capture an image of a fingerprint.

U.S. Pat. No. 7,059,203 discloses a physical sensor having a pressuresensing layer and electrical insulating layers which are integrallyformed on opposite two surfaces of the pressure sensing layer,respectively. The pressure sensing layer has a matrix comprising glassand electrically conductive particles dispersed in the matrix. One pairof electrodes are disposed on opposite sides of the pressure sensinglayer such that the electrical resistance of the pressure sensing layerbetween the electrodes is changed by application of a stress on theelectrical insulating layers. Although this sensor provides highprecision as to the amount of pressure exerted on the sensor, it cannotcapture an image of a fingerprint.

U.S. Pat. No. 7,080,562 discloses a pressure conduction sensor thatincludes a pair of locally resilient conductive layers and a locallyresilient pressure conduction composite disposed between and contactingboth conductive layers. The pressure conduction composite is composed ofa plurality of conductive particles electrically isolated within anon-conductive matrix. The conductive particles are loaded so as to havea volume fraction approaching the critical percolation threshold of thematerial system and exhibit a conductance that greatly increases withpressure. Multiple sensors that are insulated from one another can bearranged to form one or more arrays including planar and conformalconfigurations. Such an array can be used for keypads, switches andintrusion detection systems, but cannot provide the resolution necessaryto capture an image of a fingerprint.

U.S. Pat. No. 7,260,999 discloses a force sensing membrane that includesa pair of conductors and a composite material disposed between theconductors for electrically connecting the first and second conductorsunder application of sufficient pressure there between. The compositematerial contains conductive particles that have no relative orientationand are at least partially embedded in an elastomeric layer. Theconductive particles are large enough in relation to the compositematerial such that substantially all conduction paths are through singleparticles, instead of many conductive particles. The sensor cannotcapture an image of a fingerprint.

U.S. Pat. Nos. 4,856,993 and 5,033,291 and 6,964,205 disclose pressuresensors containing arrays of individual sensor elements. Each sensorelement has a pressure sensitive resistive material disposed between andattached to both of the two electrodes. U.S. Pat. No. 4,856,993discloses two sets of parallel electrodes which are each formed on athin flexible supporting sheet. The electrodes are separated by a thin,pressure sensitive resistive coating and the sensor elements are createdat the intersection points of the two sets of parallel electrodes. U.S.Pat. No. 5,033,291 also discloses two sets of parallel electrodesseparated by a thin, pressure sensitive resistive coating on a thinflexible supporting sheet, but uses adhesive dots to separate the sensorelements from one another. The adhesive dots can be used to vary thesensitivity of the array by increasing/decreasing the number of adhesivedots in an area or by creating a small space between the pressuresensitive resistive materials attached to the two electrodes. Thesensors in these two patents are created using a silk screening orprinting process. U.S. Pat. No. 6,964,205 uses a complicated design ofconductive traces and electrodes to form the sensor elements. All ofthese devices are designed to provide rather large sensor arrays tomeasure the pressure distribution between two opposing objects. Thestated resolution between contact points for these processes is 0.050inches or less. Although printing techniques have improved to providebetter sensor element density since these patents issued, theseprocesses are likely too expensive at the required resolution for use onhand-held devices, credit and debit cards, and identification badges andcards.

These previous designs do not have all the characteristics necessary forthe fingerprint biometric to be employed ubiquitously in society (i.e.,low cost, thin, mechanically conforming, mechanically flexible, adequateresolution). Accordingly, there is a need for a fingerprint sensor thatis low cost, thin, mechanically conforming, mechanically flexible andprovides adequate resolution such that it can be used on hand-helddevices, credit and debit cards, identification badges and cards, andthe like.

SUMMARY OF THE INVENTION

The present invention provides a fingerprint sensor that is low cost,thin, mechanically conforming, mechanically flexible and providesadequate resolution such that it can be used on hand-held devices,credit and debit cards, identification badges and cards, and the like.This invention discloses a novel method of manufacturing a fingerprintsensor which is mechanically flexible and costs less to manufacture thancurrently known methods. The sensor includes a compressible and flexiblecore material layered in between two orthogonally arranged electricalconductive wires. The core material's electrical resistance variesaccording to applied pressure. The orthogonal wiring arrangement allowsan external electronic circuit to successively scan each geometricintersection of the array in order to measure the local resistance. Thevariation in electrical resistance across the array corresponds to theapplied fingerprint's pattern. This invention does not use any activeelectronic components such as transistors, and does not require theapplication of any semiconductor manufacturing techniques. As a result,the device consumes significantly lower electrical power per fingerprintimage capture operation than other methods. In addition, all of themanufacturing materials and processes required to manufacture the sensordisclosed in this invention are compatible with large-scale low cost andhigh volume production. Moreover, the device is capable of sustainingmechanical stresses and long term “abuse” in a fault-tolerant manner. Asa result, the present invention enables the manufacture of very low-costfingerprint sensors for incorporation into many day-to-day consumeritems such as cell phones, credit cards, purses, wallets, etc.

One embodiment of the present invention provides a flexible pressuresensor having a composite material that includes a first set ofconductive particles, a second set of conductive particles and a thirdset of conductive particles all of which are at least partially embeddedin an elastomeric layer that is capable of returning to substantiallyits original dimensions on release of pressure. The first set ofconductive particles are electrically connected to form a first set ofsubstantially parallel conductors within a lower portion of theelastomeric layer in the x direction. The second set of conductiveparticles are electrically connected to form a second set ofsubstantially parallel conductors within an upper portion of theelastomeric layer in the y direction. The third set of conductiveparticles have no relative orientation and are disposed within theelastomeric layer for electrically connecting the first set and secondset of conductors in the z direction under application of sufficientpressure there between.

Another embodiment of the present invention provides a flexible pressuresensor having a first set of substantially parallel conductors in the xdirection, a second set of substantially parallel conductors in the ydirection, and a composite material disposed between the first set andsecond set of conductors. The composite material is capable of returningto substantially its original dimensions on release of pressure. Thecomposite material includes conductive particles at least partiallyembedded in an elastomeric layer that have no relative orientation andare disposed within the elastomeric layer for electrically connectingthe first set and second set of conductors in the z direction underapplication of sufficient pressure there between.

Yet another embodiment of the present invention provides a flexiblepressure sensor having a first set of substantially parallel conductorsin the x direction and a composite material in contact with the firstset of conductors. The composite material includes a first set ofconductive particles and a second set of conductive particles all ofwhich are at least partially embedded in an elastomeric layer that iscapable of returning to substantially its original dimensions on releaseof pressure. The first set of conductive particles are electricallyconnected to form a second set of substantially parallel conductorswithin an upper portion of the elastomeric layer in the y direction. Thesecond set of conductive particles have no relative orientation and aredisposed within the elastomeric layer for electrically connecting thefirst set and second set of conductors in the z direction underapplication of sufficient pressure there between.

Still another embodiment of the present invention provides a flexiblefingerprint sensor having a first set of substantially parallelconductors in the x direction attached to, deposited on or placed on aflexible substrate and a composite material in contact with the firstset of conductors. The composite material includes a first set ofconductive particles and a second set of conductive particles all ofwhich are at least partially embedded in an elastomeric layer that iscapable of returning to substantially its original dimensions on releaseof pressure. The first set of conductive particles are electricallyconnected to form a second set of substantially parallel conductorswithin an upper portion of the elastomeric layer in the y direction. Thesecond set of conductive particles have no relative orientation and aredisposed within the elastomeric layer for electrically connecting thefirst set and second set of conductors in the z direction underapplication of sufficient pressure there between. The conductiveparticles can be a metal, a core particle having a conductive coating,carbon nanotubes, carbon nanofibers, conductive fibers, or a combinationthereof. The first set of conductors and second set of conductorsprovide a resolution of at least 300 dots per inch and the fingerprintsensor has an overall thickness of less than 0.020 inches.

The flexible pressure sensor can be fabricated by providing a compositematerial that is capable of returning to substantially its originaldimensions on release of pressure. The composite material includesconductive particles at least partially embedded in an elastomeric layerthat have no relative orientation and are disposed within theelastomeric layer for electrically connecting a first set and a secondset of conductors in the z direction under application of sufficientpressure there between. The first set of substantially parallelconductors in the x direction are provided below or within a lowerportion of the composite material. The second set of substantiallyparallel conductors in the y direction are provided above or within anupper portion of the composite material.

The present invention is described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which:

FIG. 1 is a top view of a flexible fingerprint sensor in accordance witha first embodiment of the present invention;

FIG. 2 is a cross sectional view of the flexible fingerprint sensor ofFIG. 1;

FIG. 3 is a cross sectional view of finger on the flexible fingerprintsensor of FIG. 1;

FIG. 4 is an equivalent electrical circuit of the flexible fingerprintsensor of FIG. 1;

FIG. 5 is a flow chart showing a method for fabricating the flexiblefingerprint sensor of FIG. 1;

FIG. 6 is a top view of a flexible fingerprint sensor in accordance witha second embodiment of the present invention;

FIG. 7 is a cross sectional view of the flexible fingerprint sensor ofFIG. 6;

FIG. 8 is an equivalent electrical circuit of the flexible fingerprintsensor of FIG. 6;

FIG. 9 is a flow chart showing a method for fabricating the flexiblefingerprint sensor of FIG. 6;

FIG. 10 is a cross sectional view of a flexible fingerprint sensor inaccordance with a third embodiment of the present invention;

FIG. 11 is an equivalent electrical circuit of the flexible fingerprintsensor of FIG. 10;

FIG. 12 is a flow chart showing a method for fabricating the flexiblefingerprint sensor of FIG. 10;

FIG. 13 is a top view of a flexible fingerprint sensor in accordancewith a fourth embodiment of the present invention; and

FIG. 14 is a cross sectional view of the flexible fingerprint sensor ofFIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts thatcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and do not delimit the scope of theinvention. The discussion herein relates primarily to fingerprintsensors, but it will be understood that the concepts of the presentinvention are applicable to any medium to high resolution pressuresensor.

In current economic terms, the sensor should cost on the order of $0.25per unit in volume manufacturing in order to become acceptedubiquitously. The sensor must also be thin, as there is no physicaldepth available in most consumer products to accommodate thickcomponents. A total system-level thickness (including the mountingplatform) should be less than 0.030″ and preferably less than 0.020″.This thickness is compatible with mounting of the sensor onto a creditcard, a cell phone, and numerous other consumer devices. A typicalfingerprint sensor requires approximately a 0.5″×0.5″ active sensingarea in order to accommodate the breadth of human finger sizes, fingeralignment to the sensor, varying pressures applied, and a host ofhuman-factors related variables. This is the minimum area requirement,and a larger area will make the sensor system more reliable. Such anarea that is flat and rigid is not available on most small, hand-helddevices such as cell phones. As a result, the sensor must mechanicallyconform to the available shape (which is typically curved) of the deviceto which it is attached. In addition, the sensor, due to its physicalsize, must be able to physically flex. It is not possible to ensure arigid mounting base (irrespective of whether that base is itself flat orcontoured) and which is also physically inflexible. An example of thisrequirement is the placement of a fingerprint sensor onto a thin creditcard or drivers' license, which bend in someone's wallet, or even on thesurface of a purse which itself is not rigid.

The present invention provides a fingerprint sensor that is low cost,thin, mechanically conforming, mechanically flexible and providesadequate resolution such that it can be used on hand-held devices,credit and debit cards, identification badges and cards, and the like.This invention discloses a novel method of manufacturing a fingerprintsensor which is mechanically flexible and costs less to manufacture thancurrently known methods. The sensor includes a compressible and flexiblecore material layered in between two orthogonally arranged electricalconductive wires. The core material's electrical resistance variesaccording to applied pressure. The orthogonal wiring arrangement allowsan external electronic circuit to successively scan each geometricintersection of the array in order to measure the local resistance. Thevariation in electrical resistance across the array corresponds to theapplied fingerprint's pattern. This invention does not use any activeelectronic components such as transistors, and does not require theapplication of any semiconductor manufacturing techniques. As a result,the device consumes significantly lower electrical power per fingerprintimage capture operation than other methods. In addition, all of themanufacturing materials and processes required to manufacture the sensordisclosed in this invention are compatible with large-scale low cost andhigh volume production. Moreover, the device is capable of sustainingmechanical stresses and long term “abuse” in a fault-tolerant manner. Asa result, the present invention enables the manufacture of very low-costfingerprint sensors for incorporation into many day-to-day consumeritems such as cell phones, credit cards, purses, wallets, etc.

Now referring to FIG. 1, a top view of a flexible fingerprint sensor 100in accordance with a first embodiment of the present invention is shown.A simplified 14×14 array is shown for discussion purposes; however, apractical implementation would have at least 200-300 lines on each side.The fingerprint sensor 100 includes of a series of layers of materials.At the center is a bulk soft, compressible and elastic material, such asa polymer or an elastomer (hereinafter referred to as the base materialor composite material 102). The base material or composite material 102is impregnated with small electrically conductive particles (not shown).In other words, the conductive particles are at least partially embeddedin an elastomeric layer. The conductive particles can be metal, coreparticles having a conductive coating, carbon nanotubes, carbonnanofibers, conductive fibers, or a combination thereof. The conductiveparticles are distributed homogeneously with no relative orientationwithin the composite material 102 in a bulk fashion, such as in amelting and mixing process. Composite materials 102 having conductiveparticles are disclosed in the following U.S. patents, all of which areincorporated by reference in their entirety: U.S. Pat. Nos. 4,644,101;6,915,701; 7,059,203; 7,080,562; and 7,260,999.

The composite material 102 is made into a thin sheet and cut intofingerprint sensor sizes (typically 0.5″×0.5″). A typical thickness ofthis film is on the order of 0.010″ to 0.030″. The density of theconductive materials, as well as the size distribution of theseparticulates, is made so that in the normal state (when no pressure isapplied) of the thin film, the electrical resistance from top to bottomsurfaces is large (on the order of one or more Mega-ohms). However, withthe application of a small force onto the surface of this impregnatedbase material, the resistance will drop significantly, due to thepressing of the conductive particulates together forming a current pathfrom top surface to bottom surface. Since the composite material 102 isthin, the deformation in the vertical dimension (the z direction) issmall, and thus an adequate resolution in the orthogonal dimensions(where the finger rests onto the sensor, x and y directions) isachievable—enough to sense the individual ridges of human fingers. Therequired resolution in order to sense the variety of human fingerprintsacross the worldwide population is approximately 300 dots per inch, or0.033″ in both dimensions.

One mechanism to transfer the resistance change of the compositematerial 102 to a measurement device (such as a voltage source and avoltage measurement circuit) is achieved by connecting a first set ofsubstantially parallel electrical conductors 104 in the x direction onthe bottom or lower portion of the composite material 102, and a secondset of substantially parallel electrical conductors 106 in the ydirection on the top or upper portion of the composite material 102. Thefirst set of conductors 104 and the second set of conductors 106 form anarray or matrix of rows and columns on the top and bottom surfaces ofthe composite material 102. The geometric intersection of the x and ywires at each point (e.g., 108) allows an external device to measure thevertical resistance (in the z direction) of the composite material 102at that particular pixel location. A scan of the whole array ofresistive elements (each formed at the x and y intersection, at eachpixel) provides a complete resistive image of the fingerprint.Typically, the sets of parallel electrical conductors (e.g., 104) can beprinted or etched (or otherwise deposited) onto a flexible substrate(e.g., 110) such as a polyimide film. Two of these films can be usedorthogonally (one in the x direction and one in the y direction), withthe composite material 102 sandwiched in the middle. Note that the upperflexible substrate is not shown. The alignment of the composite material102 to the x and y wiring layers is not critical, as the impregnation ofthe conductors into the base material is homogeneous and not patterned.Very gross alignment is sufficient. As a result, eliminating thenecessity of creating specific sensor elements within the compositematerial 102 reduces cost, improves yield, and improves performance andreliability because the sensor points are created wherever the sets ofconductors intersect. The sensor can be integrated into a hand-helddevice, a credit card, a debit card, an identification badge, anidentification card, an access card or a passport. Techniques to processor otherwise use the data produced by the sensor are well known and neednot be discussed herein.

Referring now to FIG. 2, a cross sectional view of the flexiblefingerprint sensor 100 of FIG. 1 is shown. The composite material 102 isdisposed between a first set of substantially parallel conductors 104 inthe x direction and a second set of substantially parallel conductors106 in the y direction. The composite material 102 is capable ofreturning to substantially its original dimensions on release ofpressure. The composite material 102 includes conductive particles 200at least partially embedded in an elastomeric layer that have norelative orientation and are disposed within the elastomeric layer forelectrically connecting the first set 104 and second set 106 ofconductors in the z direction under application of sufficient pressurethere between. Note the conductive particles 200 are typically of small(but not identical) sizes homogeneously suspended in the compositematerial 102. The first set of parallel conductors 104 are attached to,deposited on or placed on a first (lower) flexible substrate 110.Similarly, the second set of parallel conductors 106 are attached to,deposited on or placed on a second (upper) flexible substrate 202.

Now referring to FIG. 3, a cross sectional view of finger 300 on theflexible fingerprint sensor 100 of FIG. 1 is shown. In operation, thesecond (upper) flexible substrate or membrane 202 bends under pressurefrom the finger ridge 302, which compresses the composite material 102.The compression causes the conductive particles 200 to mechanicallycontact, causing a short circuit (denoted by arrow 304) (or at least adrop in electrical resistance) from the top surface 306 to the bottomsurface 308. This change in electrical resistance is localized to wherethe finger ridges 302 are located. These locations are then reflected aslow-resistance readings as the rows (x) and columns (y) are scanned inreading and recording their resistance. Since the base material is bothflexible and elastomeric (like a rubber ball), it returns to its inertshape (flat and high resistance everywhere) after finger pressure isremoved.

Referring now to FIG. 4, an equivalent electrical circuit 400 of theflexible fingerprint sensor 100 of FIG. 1 is shown. The electricalcircuit 400 shows three pixels 402, 404 and 406 on the same row 408. Themiddle pixel 404 is compressed by a finger ridge causing its electricalresistance from (R) to drop to (r). The two adjacent pixels 402 and 406are not compressed, so their resistance (R) remains high. These adjacentpixels' resistance might drop a little due to their being adjacent tothe pressure point, but there is still discernable differential inresistance to indicate the center point of the ridge maximum pressure(and thus the location of the ridge). Note that these three pixels 402,404 and 406 share one common electrical node 408 (the bottom trace), butare isolated in their upper nodes. In this manner, it is possible toelectrically read each resistor's value individually. When no finger ispushing against the sensor, all resistors are at a high value (R). Whena finger is pressed onto the sensor, the ridges of the finger willcreate a greater pressure onto their respective x-y locations, causingthese locations to exhibit a lower resistance (r). Thus the physicalfingerprint ridges are converted into a set of resistance values in thex-y dimensions which are digitized for further image processing.

Now referring to FIG. 5, a flow chart showing a method 500 forfabricating the flexible fingerprint sensor 100 of FIG. 1 is shown. Theflexible pressure sensor 100 can be fabricated by providing a first(lower) flexible substrate or layer 110 in block 502 and attaching,depositing or placing a first (lower) set of substantially parallelconductors 104 in the x direction on the first (lower) flexible layer110 in block 504. A composite material 102 that is capable of returningto substantially its original dimensions on release of pressure isattached, deposited or placed on the first (lower) set of conductors 104in block 506. The composite material 102 includes conductive particlesat least partially embedded in an elastomeric layer that have norelative orientation and are disposed within the elastomeric layer forelectrically connecting the first set 104 and the second set 106 ofconductors in the z direction under application of sufficient pressurethere between. A second (upper) set of substantially parallel conductors106 in the y direction are attached, deposited or placed on thecomposite material 102 in block 508. A second (upper) flexible substrateor layer 202 is then attached, deposited or placed on the second (upper)set of conductors 106 in block 510. Note that steps 502 and 504(likewise 508 and 510) can be combined such that the flexible layer andelectrical conductors can be prefabricated. Conductive leads are thenattached to the first set of conductors 104 and the second set ofconductors 106 in block 512. Additional steps may also be preformed,such as: providing one or more additional layers 514; encapsulating thesensor in a package 514; or integrating the sensor into a hand-helddevice, a credit card, a debit card, an identification badge, anidentification card, an access card or a passport.

Referring now to FIGS. 6 and 7, a top view (FIG. 6) and a cross sectionview (FIG. 7) of a flexible fingerprint sensor 600 in accordance with asecond embodiment of the present invention are shown. In thisembodiment, it is possible to eliminate the top-most layer of conductorcarrying material. This is useful in order to further increase thespatial resolution of the sensor by eliminating any special bufferingthat this top layer exhibits. The column (y-direction) conductors arestill required in order for the electrical scanning circuit to becompleted. The flexible pressure sensor 600 has a first set ofsubstantially parallel conductors 104 in the x direction and a compositematerial 102 in contact with the first set of conductors 104. Thecomposite material 102 includes a first set of conductive particles 602and a second set of conductive particles 200 all of which are at leastpartially embedded in an elastomeric layer that is capable of returningto substantially its original dimensions on release of pressure. Thefirst set of conductive particles 602 are electrically connected to forma second set of substantially parallel conductors 604 within an upperportion of the elastomeric layer in the y direction. Note that thesecond set of conductors 602 can be semi-conductive or partiallyconductive as along as a change in resistance can be detected when afinger presses on the sensor. The second set of conductors 602 can beformed using a laser beam or other directed heat source which scansabove the material in the y-direction, causing localized melting andrecombination of these particulates into a series of electricallyconductive lines. It is important that the depth of the recombination informing this conductive line not be too deep and make contact to the rowconductors, as a vertical short circuit is made. The second set ofconductive particles 200 have no relative orientation and are disposedwithin the elastomeric layer for electrically connecting the first set104 and second set 604 of conductors in the z direction underapplication of sufficient pressure there between. The flexible pressuresensor 600 may include one or more additional layers (e.g., 110) thatprovide protection against contamination or physical damage, adhesion, amechanical mounting structure, a flexible substrate or a combinationthereof. The composite material 102 can also be encapsulated in aprotective material having a first set of conductive leads electricallyconnected to the first set of conductors 104 that extend through theprotective material and a second set of conductive leads electricallyconnected to the second set of conductors 604 that extend through theprotective material.

Referring now to FIG. 8, an equivalent electrical circuit of theflexible fingerprint sensor 600 of FIG. 6 is shown. The electricalcircuit 800 shows three pixels 802, 804 and 806 on the same row 808.Since the second set of conductors 604 are created from the conductiveparticles, the resistance (r1) of these conductors will be higher thanthe resistance of the first set of conductors (e.g., 808). Thecumulative resistance (r1+r2) needs to be sufficiently lower than (r1+R)so that the change can be detected. The middle pixel 804 is compressedby a finger ridge causing its electrical resistance to drop from (R) to(r2). The two adjacent pixels 802 and 806 are not compressed, so theirresistance (R) remains high. These adjacent pixels' resistance mightdrop a little due to their being adjacent to the pressure point, butthere is still discernable differential in resistance to indicate thecenter point of the ridge maximum pressure (and thus the location of theridge). Note that these three pixels 802, 804 and 806 share one commonelectrical node 808 (the bottom trace), but are isolated in their uppernodes. In this manner, it is possible to electrically read eachresistor's value individually. When no finger is pushing against thesensor, all resistors are at a high value (R). When a finger is pressedonto the sensor, the ridges of the finger will create a greater pressureonto their respective x-y locations, causing these locations to exhibita lower resistance (r2). Thus the physical fingerprint ridges areconverted into a set of resistance values in the x-y dimensions whichare digitized for further image processing.

Now referring to FIG. 9, a flow chart showing a method 900 forfabricating the flexible fingerprint sensor 600 of FIG. 6 is shown. Theflexible pressure sensor 600 can be fabricated by providing a first(lower) flexible substrate or layer 110 in block 902 and attaching,depositing or placing a first (lower) set of substantially parallelconductors 104 in the x direction on the first (lower) flexible layer110 in block 904. A composite material 102 that is capable of returningto substantially its original dimensions on release of pressure isattached, deposited or placed on the first (lower) set of conductors 104in block 906. The composite material 102 includes conductive particlesat least partially embedded in an elastomeric layer that have norelative orientation and are disposed within the elastomeric layer forelectrically connecting the first set 104 and the second set 604 ofconductors in the z direction under application of sufficient pressurethere between. A second (upper) set of substantially parallel conductors604 in the y direction are created by electrically connecting conductiveparticles 602 within the composite material 102 in block 908. Note thatsteps 902 and 904 can be combined such that the flexible layer andelectrical conductors can be prefabricated. Similarly, steps 906 and 908can be combined such that the electrical conductors within the compositematerial can be prefabricated. Conductive leads are then attached to thefirst set of conductors 104 and the second set of conductors 604 inblock 910. Additional steps may also be preformed, such as: providingone or more additional layers 912; encapsulating the sensor in a package912; or integrating the sensor into a hand-held device, a credit card, adebit card, an identification badge, an identification card, an accesscard or a passport.

Referring now to FIG. 10, a cross sectional view of a flexiblefingerprint sensor 1000 in accordance with a third embodiment of thepresent invention is shown. The flexible pressure sensor 1000 has acomposite material 102 that includes a first set of conductive particles1002, a second set of conductive particles 602 and a third set ofconductive particles 200 all of which are at least partially embedded inan elastomeric layer that is capable of returning to substantially itsoriginal dimensions on release of pressure. The first set of conductiveparticles 1002 are electrically connected to form a first set ofsubstantially parallel conductors 1004 within a lower portion of theelastomeric layer in the x direction. The second set of conductiveparticles 602 are electrically connected to form a second set ofsubstantially parallel conductors 604 within an upper portion of theelastomeric layer in the y direction. The third set of conductiveparticles 200 have no relative orientation and are disposed within theelastomeric layer for electrically connecting the first set 1004 andsecond set 604 of conductors in the z direction under application ofsufficient pressure there between. The flexible pressure sensor 1000 mayinclude one or more additional layers (e.g., 110) that provideprotection against contamination or physical damage, adhesion, amechanical mounting structure, a flexible substrate or a combinationthereof. The composite material 102 can also be encapsulated in aprotective material having a first set of conductive leads electricallyconnected to the first set of conductors 1004 that extend through theprotective material and a second set of conductive leads electricallyconnected to the second set of conductors 604 that extend through theprotective material.

Now referring to FIG. 11, an equivalent electrical circuit of theflexible fingerprint sensor 1000 of FIG. 10 is shown. The electricalcircuit 1100 shows three pixels 1102, 1104 and 1106 on the same row1108. The first set of conductors are created from conductive particles1002 and have a resistance (r3). The second set of conductors 604 arecreated from conductive particles 602 and have a resistance (r1). Thecumulative resistance (r1+r2+r3) needs to be sufficiently lower than(r1+R+r3) so that the change can be detected. The middle pixel 1104 iscompressed by a finger ridge causing its electrical resistance to dropfrom (R) to (r2). The two adjacent pixels 1102 and 1106 are notcompressed, so their resistance (R) remains high. These adjacent pixels'resistance might drop a little due to their being adjacent to thepressure point, but there is still discernable differential inresistance to indicate the center point of the ridge maximum pressure(and thus the location of the ridge). Note that these three pixels 1102,1104 and 1106 share one common electrical node 1108 (the bottom trace),but are isolated in their upper nodes. In this manner, it is possible toelectrically read each resistor's value individually. When no finger ispushing against the sensor, all resistors are at a high value (R). Whena finger is pressed onto the sensor, the ridges of the finger willcreate a greater pressure onto their respective x-y locations, causingthese locations to exhibit a lower resistance (r2). Thus the physicalfingerprint ridges are converted into a set of resistance values in thex-y dimensions which are digitized for further image processing.

Referring now to FIG. 12, a flow chart showing a method 1200 forfabricating the flexible fingerprint sensor 1000 of FIG. 10 is shown.The flexible pressure sensor 1000 can be fabricated by providing a first(lower) flexible substrate or layer 110 in block 1202. A first (lower)set of substantially parallel conductors 1004 in the y direction arecreated by electrically connecting conductive particles 1002 within thecomposite material 102 in block 1204. A composite material 102 that iscapable of returning to substantially its original dimensions on releaseof pressure is attached, deposited or placed on the first (lower)flexible substrate 110 in block 1206. The composite material 102includes conductive particles at least partially embedded in anelastomeric layer that have no relative orientation and are disposedwithin the elastomeric layer for electrically connecting the first set1004 and the second set 604 of conductors in the z direction underapplication of sufficient pressure there between. A second (upper) setof substantially parallel conductors 604 in the y direction are createdby electrically connecting conductive particles 602 within the compositematerial 102 in block 1208. Note that steps 1204 and 1206 can becombined such that the both sets of electrical conductors can beprefabricated in the composite material 102. Conductive leads are thenattached to the first set of conductors 1004 and the second set ofconductors 604 in block 1210. Additional steps may also be preformed,such as: providing one or more additional layers 1212; encapsulating thesensor in a package 1212; or integrating the sensor into a hand-helddevice, a credit card, a debit card, an identification badge, anidentification card, an access card or a passport.

Now referring to FIGS. 13 and 14, a top view (FIG. 13) and a crosssectional view (FIG. 14) of a flexible fingerprint sensor 1300 inaccordance with a fourth embodiment of the present invention is shown.In this embodiment, the flexible pressure sensor 1300 has a first set ofsubstantially parallel conductors 104 or 1004 in the x direction and acomposite material 102 in contact with the first set of conductors 104or 1004. The composite material 102 includes a first set of conductiveparticles 200 and a fourth set of conductive particles 1302 all of whichare at least partially embedded in an elastomeric layer that is capableof returning to substantially its original dimensions on release ofpressure. The fourth set of conductive particles 1302 are electricallyconnected to form a third set of substantially parallel conductors 1304(similar to vias) in the z direction to connect the second set ofconductors 106 or 604 to a set of conductive leads 1306 within orattached to the lower portion of the elastomeric layer. Note that thethird set of conductors 1302 can be semi-conductive or partiallyconductive as along as a change in resistance can be detected when afinger presses on the sensor. The third set of conductors 1304 can beformed using a laser beam or other directed heat source which scans thematerial in the z-direction, causing localized melting and recombinationof these particulates into a series of electrically conductive lines.The second set of conductive particles 200 have no relative orientationand are disposed within the elastomeric layer for electricallyconnecting the first set 104 or 1004 and second set 106 or 604 ofconductors in the z direction under application of sufficient pressurethere between. The flexible pressure sensor 1300 may include one or moreadditional layers (e.g., 110) that provide protection againstcontamination or physical damage, adhesion, a mechanical mountingstructure, a flexible substrate or a combination thereof. The compositematerial 102 can also be encapsulated in a protective material having afirst set of conductive leads electrically connected to the first set ofconductors 104 or 1004 that extend through the protective material and asecond set of conductive leads electrically connected to the third setof conductors 1306 that extend through the protective material.

Although preferred embodiments of the present invention have beendescribed in detail, it will be understood by those skilled in the artthat various modifications can be made therein without departing fromthe spirit and scope of the invention as set forth in the appendedclaims.

1. A flexible pressure sensor comprising: a single elastomeric layer having a top, a bottom, a lower portion above the bottom, an upper portion below the top, and a middle portion between the lower portion and the upper portion, wherein the single elastomeric layer is capable of returning to substantially its original dimensions on release of pressure; a plurality of non-oriented conductive particles distributed homogeneously with no relative orientation between the top and bottom of the single elastomeric layer; a first set of the non-oriented conductive particles within the single elastomeric layer that have been combined together to form a first set of substantially parallel conductors disposed within the lower portion of the single elastomeric layer above the bottom of the single elastomeric layer in the x direction; a second set of the non-oriented conductive particles within the single elastomeric layer that have been combined together to form a second set of substantially parallel conductors disposed within the upper portion of the single elastomeric layer below the top of the single elastomeric layer in the y direction; and wherein the non-oriented conductive particles disposed within the single elastomeric layer electrically connect the first set and second set of conductors in the z direction under application of sufficient pressure that reduces an electrical resistance of the single elastomeric layer there between.
 2. The flexible pressure sensor as recited in claim 1, further comprising: a first layer disposed above the top of the single elastomeric layer; or a second layer disposed below the bottom of the single elastomeric layer.
 3. The flexible pressure sensor as recited in claim 2, wherein the first and second layers provide protection against contamination or physical damage, adhesion, a mechanical mounting structure, a flexible substrate or a combination thereof.
 4. The flexible pressure sensor as recited in claim 1, wherein the single elastomeric layer is encapsulated in a protective material having a first set of conductive leads electrically connected to the first set of conductors that extend through the protective material and a second set of conductive leads electrically connected to the second set of conductors that extend through the protective material.
 5. The flexible pressure sensor as recited in claim 1, further comprising a third set of the non-oriented conductive particles within the single elastomeric layer that have been combined together to form a third set of substantially parallel conductors disposed within the lower portion, the middle portion and the upper portion of the single elastomeric layer in the z direction between the top and the bottom of the single elastomeric layer to permanently connect the second set of conductors to a set of conductive leads within or attached to the lower portion of the single elastomeric layer.
 6. The flexible pressure sensor as recited in claim 1, wherein the flexible pressure sensor is a fingerprint sensor.
 7. The flexible pressure sensor as recited in claim 1, wherein the flexible pressure sensor has an overall thickness of less than 0.030 inches.
 8. The flexible pressure sensor as recited in claim 1, wherein the flexible pressure sensor has an overall thickness of less than 0.020 inches.
 9. The flexible pressure sensor as recited in claim 1, wherein the first set of conductors and second set of conductors provide a resolution of at least 200 dots per inch.
 10. The flexible pressure sensor as recited in claim 1, wherein the first set of conductors and second set of conductors provide a resolution of at least 300 dots per inch.
 11. The flexible pressure sensor as recited in claim 1, wherein the conductive particles comprise a metal, a core particle having a conductive coating, carbon nanotubes, carbon nanofibers, conductive fibers, or a combination thereof.
 12. The flexible pressure sensor as recited in claim 1, wherein the sensor is integrated into a hand-held device, a credit card, a debit card, an identification badge, an identification card, an access card or a passport.
 13. A flexible pressure sensor comprising: a first set of substantially parallel conductors in the x direction; a single elastomeric layer having a top and a bottom such that the bottom of the single elastomeric layer is in contact with the first set of conductors, wherein the single elastomeric layer: (a) has a lower portion above the bottom, an upper portion below the top, and a middle portion between the lower portion and the upper portion, (b) is capable of returning to substantially its original dimensions on release of pressure and (c) comprises a plurality of non-oriented conductive particles distributed homogeneously with no relative orientation between the top and the bottom of the single elastomeric layer; a first set of the non-oriented conductive particles within the single elastomeric layer that have been combined together to form a second set of substantially parallel conductors disposed within the upper portion of the single elastomeric layer below the top of the single elastomeric layer in the y direction; and wherein the non-oriented conductive particles disposed within the single elastomeric layer electrically connect the first set and second set of conductors in the z direction under application of sufficient pressure that reduces an electrical resistance of the single elastomeric layer there between.
 14. The flexible pressure sensor as recited in claim 13, further comprising one or more additional layers that provide protection against contamination or physical damage, adhesion, a mechanical mounting structure, a flexible substrate or a combination thereof.
 15. The flexible pressure sensor as recited in claim 13, wherein the single elastomeric layer is encapsulated in a protective material having a first set of conductive leads electrically connected to the first set of conductors that extend through the protective material and a second set of conductive leads electrically connected to the second set of conductors that extend through the protective material.
 16. The flexible pressure sensor as recited in claim 13, further comprising a second set of the non-oriented conductive particles within the single elastomeric layer that have been combined together to form a third set of substantially parallel conductors disposed within the lower portion, the middle portion and the upper portion of the single elastomeric layer in the z direction between the top and the bottom of the single elastomeric layer to connect the second set of conductors to a set of conductive leads within or attached to the lower portion of the single elastomeric layer.
 17. The flexible pressure sensor as recited in claim 13, wherein the flexible pressure sensor is a fingerprint sensor.
 18. The flexible pressure sensor as recited in claim 13, wherein the flexible pressure sensor has an overall thickness of less than 0.030 inches.
 19. The flexible pressure sensor as recited in claim 13, wherein the flexible pressure sensor has an overall thickness of less than 0.020 inches.
 20. The flexible pressure sensor as recited in claim 13, wherein the first set of conductors and second set of conductors provide a resolution of at least 200 dots per inch.
 21. The flexible pressure sensor as recited in claim 13, wherein the first set of conductors and second set of conductors provide a resolution of at least 300 dots per inch.
 22. The flexible pressure sensor as recited in claim 13, wherein the conductive particles comprise a metal, a core particle having a conductive coating, carbon nanotubes, carbon nanofibers, conductive fibers, or a combination thereof.
 23. The flexible pressure sensor as recited in claim 13, wherein the sensor is integrated into a hand-held device, a credit card, a debit card, an identification badge, an identification card, an access card or a passport.
 24. A flexible fingerprint sensor comprising: a first set of substantially parallel conductors in the x direction attached to, deposited on or placed on a flexible substrate; a single elastomeric layer having a top and a bottom such that the bottom of the single elastomeric layer is in contact with the first set of conductors, wherein the single elastomeric layer: (a) has a lower portion above the bottom, an upper portion below the top, and a middle portion between the lower portion and the upper portion, (b) is capable of returning to substantially its original dimensions on release of pressure and (c) comprises a plurality of non-oriented conductive particles distributed homogeneously with no relative orientation between the top and the bottom of the single elastomeric layer; a first set of the non-oriented conductive particles within the single elastomeric layer that have been combined together to form a second set of substantially parallel conductors disposed within the upper portion of the single elastomeric layer below the top of the single elastomeric layer in the y direction; and wherein the non-oriented conductive particles disposed within the single elastomeric layer electrically connect the first set and second set of conductors in the z direction under application of sufficient pressure that reduces an electrical resistance of the single elastomeric layer there between.
 25. The flexible fingerprint sensor as recited in claim 24, wherein the conductive particles comprise a metal, a core particle having a conductive coating, carbon nanotubes, carbon nanofibers, conductive fibers, or a combination thereof.
 26. The flexible fingerprint sensor as recited in claim 24, wherein the flexible pressure sensor has an overall thickness of less than 0.030 inches.
 27. The flexible fingerprint sensor as recited in claim 24, wherein the flexible pressure sensor has an overall thickness of less than 0.020 inches.
 28. The flexible fingerprint sensor as recited in claim 24, wherein the first set of conductors and second set of conductors provide a resolution of at least 200 dots per inch.
 29. The flexible fingerprint sensor as recited in claim 24, wherein the first set of conductors and second set of conductors provide a resolution of at least 300 dots per inch.
 30. The flexible fingerprint sensor as recited in claim 24, wherein the sensor is integrated into a hand-held device, a credit card, a debit card, an identification badge, an identification card, an access card or a passport.
 31. The flexible fingerprint sensor as recited in claim 24, further comprising a second set of the non-oriented conductive particles within the single elastomeric layer that have been combined together to form a third set of substantially parallel conductors disposed within the lower portion, the middle portion and the upper portion of the single elastomeric layer in the z direction between the top and the bottom of the single elastomeric layer to connect the second set of conductors to a set of conductive leads within or attached to the lower portion of the single elastomeric layer.
 32. A method for fabricating a flexible pressure sensor comprising the steps of: providing a single elastomeric layer having a top, a bottom, a lower portion above the bottom, an upper portion below the top, and a middle portion between the lower portion and the upper portion, wherein the single elastomeric layer is capable of returning to substantially its original dimensions on release of pressure and comprises a plurality of non-oriented conductive particles distributed homogeneously with no relative orientation between the top and the bottom of the single elastomeric layer; creating a first set of substantially parallel conductors within the single elastomeric layer in the x direction disposed within the lower portion of the single elastomeric layer above the bottom of the single elastomeric layer using only the non oriented conductive particles by localized melting and recombination of a first set of the non-oriented conductive particles at least partially embedded in the lower portion of the elastomeric layer in the x direction; creating a second set of substantially parallel conductors within the single elastomeric layer in the y direction disposed within the upper portion of the single elastomeric layer below the top of the single elastomeric layer using only the non oriented conductive particles by localized melting and recombination of a second set of the non-oriented conductive particles at least partially embedded in the upper portion of the elastomeric layer in the y direction; and wherein the non-oriented conductive particles embedded in the elastomeric layer electrically connect the first set conductors to the second set of conductors in the z direction under application of sufficient pressure that reduces an electrical resistance of the single elastomeric layer there between.
 33. The method as recited in claim 32, wherein the flexible pressure sensor is a fingerprint sensor.
 34. The method as recited in claim 32, wherein the flexible pressure sensor has an overall thickness of less than 0.030 inches.
 35. The method as recited in claim 32, wherein the flexible pressure sensor has an overall thickness of less than 0.020 inches.
 36. The method as recited in claim 32, wherein the first set of conductors and second set of conductors provide a resolution of at least 200 dots per inch.
 37. The method as recited in claim 32, wherein the first set of conductors and second set of conductors provide a resolution of at least 300 dots per inch.
 38. The method as recited in claim 32, wherein the conductive particles comprise a metal, a core particle having a conductive coating, carbon nanotubes, carbon nanofibers, conductive fibers, or a combination thereof.
 39. The method as recited in claim 32, further comprising the step of integrating the flexible pressure sensor into a hand-held device, a credit card, a debit card, an identification badge, an identification card, an access card or a passport.
 40. The method as recited in claim 32, wherein the first and second set of parallel conductors are semi-conductive or partially conductive.
 41. The method as recited in claim 32, further comprising the step of creating a third set of substantially parallel conductors within the single elastomeric layer in the z direction between the top and the bottom of the single elastomeric layer by electrically connecting a fourth set of conductive particles within the lower portion, the middle portion and the upper portion of the single elastomeric layer between the second set of conductors and a set of conductive leads within or attached to the lower portion of the elastomeric layer by localized melting and permanent recombination of the fourth set of conductive particles.
 42. The method as recited in claim 32, further comprising the step of attaching conductive leads to the first set of conductors and the second set of conductors.
 43. The method as recited in claim 22, further comprising the step of providing one or more additional layers.
 44. The method as recited in claim 32, further comprising the step of encapsulating the sensor in a package. 